Phyllosilicates (e.g., montmorillonites, hectorite, saponite, etc.) and synthetic phyllosilicates (e.g., LAPONITE) are used as rheology control agents in a number of applications such as paints, drilling fluids, adhesives, greases, cosmetics, foods, pharmaceuticals, inks, etc. The thickening ability of natural and synthetic smectites is due to the interactions between the phyllosilicate platelets that lead to a “house of cards” structure. The association of phyllosilicate particles occurs primarily through edge/edge interactions which generate ribbon-like microstructures. Because these particle/particle interactions are relatively weak, the colloidal structure of the phyllosilicate particles is easily disrupted by shear forces, leading to time-and shear-dependent rheology, or thixotropy. Thixotropic rheology is desirable in numerous applications. For example, thixotropy in coatings imparts high viscosity, thereby preventing pigment settling, but permits easy flow of the coating during application. The time dependent behavior of the viscosity enables film leveling but prevents sag.
Due to their extremely small particle size, smectites can be very efficient thickening agents. For example, phyllosilicate concentrations as low as 0.4 to 0.8 weight percent weight percent are sufficient to build acceptable viscosity in latex paints. Aqueous dispersions at solids concentrations as low as 4 to 8 weight percent produce highly viscous pastes. Even at these low solid concentrations the high viscosity build severely limits the concentrations at which smectite slurries can be handled during, for example, beneficiation processes.
Accordingly, the use of dispersing agents to limit viscosities in phyllosilicate slurries is well known. Examples of some inorganic dispersants that have found wide use in phyllosilicate processing applications include tetrasodium pyrophosphate (TSPP), sodium tripolyphosphate (STPP), hexametaphosphate, and sodium silicate. These materials function primarily through adsorption at the edges of phyllosilicate platelets, but can also complex multi-charged cations such as calcium and magnesium which can otherwise lead to phyllosilicate flocculation. Organic dispersing agents such as tannates, humates, and synthetic polymers such as polyacrylates, polyethylene oxide, polyvinyl alcohol, etc. function by preventing particle/particle interactions through steric repulsion.
The use of organophosphorus reagents as dispersants of synthetic phyllosilicates is known in the art. Organophosphorous reagents include phosphonic acids, phosphinic acids, thiophosphinic acids, diesters of phosphorous acid, diesters of phosphoric acid, and diphosphonic acids. These dispersants permit highly concentrated suspensions of synthetic phyllosilicates to be prepared that exhibit extremely low viscosities. Additionally, these dispersants do not appear to interfere with the effectiveness of the synthetic phyllosilicate as a thickening agent in finished products such as paints.
Although dispersants reduce the viscosity of phyllosilicate slurries, current dispersants do not provide phyllosilicate compositions with other desirable properties including corrosion inhibition and the ability to selectively control the viscosity of a phyllosilicate dispersion. Additionally, the prior art use of organophosphorous dispersants is disadvantageous because these dispersants interfere with the generation of high viscosity dispersions in nonaqueous systems. It is believed that organophosphorous dispersants interfere with edge-edge interactions of the phyllosilicate particles which are theorized to be responsible for increased viscosity.
Objects and advantages of the present invention will become apparent by review of the detailed description of preferred embodiments.